Lots of opportunities in what they call the Northern Tier of PA. In the Mansfield and Wellsboro area there is the TwistedSpokes.org group, with what looks like weekly rides. They ride primarily in the Tioga State Forest lands.

South of Mansfield, roughly between there and Williamsport, there is the Tiadaghton State Forest, with many options, and west of Wellsboro to Coudersport there is additional riding, including lift served at Denton Hill State Park/Ski Area.

There's a link in this thread to GPS tracks of MTB rides in and around Tioga County (ie., Mansfield and Wellsboro). In the Starting Near field on that Garmin map site, try 'Potter County', or Corning, NY and you'll see even more options.

I do STPR every year as well and actually have a cabin on Stages 8-10 on 12 mile road. Awesome.
Still trying to learn some of the trails too, but trail maintenance must be somewhat difficult just because it is such a huge area.